The invention relates to an image recording apparatus such as a laser printer in which a beam from a laser source is deflected by a deflector such as a rotating polygon mirror and a surface to be scanned is scanned by such deflected beam. More particularly, the invention is directed to an apparatus in which the beam intensity can be varied in accordance with the scanning density or the like.
A conventional apparatus for writing an image by scanning a surface to be scanned such as a photoreceptor drum using a beam applied from a laser source includes: a printer called a laser beam printer and a digital electronic copying machine. For example, a laser beam printer 1 such as shown in FIG. 4 is used as a typical example of the above-mentioned image forming apparatus. In the laser beam printer 1 shown in FIG. 4, a laser scanner 2 is used as an image writing unit. This laser scanner 2, using a laser beam output member 3 that outputs a laser beam while modulating it in accordance with an image signal, causes the laser beam outputted from the laser beam output member 3 to be irradiated to a polygon mirror 4. Then, the laser beam deflected in accordance with the rotation of the polygon mirror 4 is caused to pass through an f.theta. lens 5 and to change its advancing direction through mirrors 6 and 7, and is outputted to a photoreceptor drum 10. Below the laser scanner 2, the photoreceptor drum 10 that rotates at a predetermined speed is provided, and the laser beam outputted from the laser scanner 2 is used to scan the photoreceptor drum 10 at an exposing position 12 in the width direction of the drum (main scanning direction) repetitively. Upstream to the exposing position of the photoreceptor drum 10 is a charger 11, while downstream from the exposing position is a developing unit 13. The charger 11 charges the surface of the photoreceptor drum 10 uniformly, and a latent electrostatic image is formed with the irradiation of the laser beam from the laser scanner 2 at the exposing position. Toner is supplied from the developing unit 13 onto the latent electrostatic image to form a toner image, and the toner image is transferred onto a sheet with discharging operation by a transfer corotron 24. The charger 11 and the transfer corotron 24 are formed of a single-strand corotron wire stretched over a shielded space with a voltage applying terminal being provided on one end of the wire.
The toner or the like remaining on the surface of the photoreceptor drum 10 after the toner image has been transferred onto the sheet from the drum 10 in such a manner as described above is wiped away by a blade 17 of a cleaner 16, and then the drum 10 is charged again by the charger 11 so that a next image can be formed. The toner or the like wiped by the blade 17 is forwarded to a toner recovery hopper by a forwarding unit 18 such as an auger or the like disposed inside the cleaner 16. Further, the developing unit 13 is provided with a toner container section having a stirring means 14 and a developing roller 15, so that the surface of the developing roller 15 with the toner adhering thereon is rotated toward the latent electrostatic image over a portion corresponding to the photoreceptor drum 10. Further, the developing bias is applied to the developing roller 15 by a not shown power supply means.
The sheet on which the toner image formed on the photoreceptor drum is transferred is supplied from a sheet feed cassette 20 accommodated in the lower portion of the apparatus. As the sheet feed cassette 20, a cassette having a structure similar to that of an ordinarily used tray may be employed. The sheet contained in the sheet feed cassette 20 is forwarded by a sheet feed roller 21 which is a semicircular roller and further forwarded by a forward roller unit 22 disposed in a sheet forward path. On the portion immediately before the image transfer position in the sheet forward path, a registration roller unit 23 is provided. The roller unit 23 temporarily stops the sheet with the front end of the sheet nipped and is driven in synchronism with the timing of the toner image formed on the photoreceptor drum. The toner image on the photoreceptor drum is then transferred onto the sheet by discharging operation of the transfer corotron 24.
The sheet on which the toner image has been transferred is guided into a fuser 25, and when passing between a heat roller 26 and a pressure roller 27 provided at the fuser 25, the sheet is subjected to heat and pressure to fuse the toner and fix the toner thereon, thus producing a copy. The copy discharged from the fuser 25 is forwarded either into a discharge tray that is arranged on the side or into an upper discharge tray 34 by selecting a path with a switching member 31 disposed on a discharge path 30. Discharge rollers 32 and 33 are provided so as to correspond to the above-mentioned two discharge trays, the discharge tray on the side discharging copies with the sheets faceup, while the upper discharge tray 34 discharging the copies with the sheets facedown with pagination.
The thus constructed laser beam printer 1 has a control circuit such as shown in FIG. 5. The laser beam printer 1 shown in FIG. 4 includes a controller 40 mounting a central processing unit (CPU), the control of the controller 40 is effected in accordance with a program stored in a built-in read only memory. Various circuit components are connected to the controller 40 shown in FIG. 5. For example, sensors 41 for sensing sheet forward conditions, a drive unit 45 such as a main motor for driving mechanical components of various units, control means 42 such as various clutches and solenoids that control the drive force transmitted from the drive unit 45, and the like.
Further, with respect to the discharging unit such as the charger and the transfer corotron, a high-voltage power generator 44, a power supply control means for the fuser 25, a control means for the laser scanner 2, and the like are also connected to the controller 40. The controller 40 is connected to a not shown image information processor such as a computer using a cable 47 having a connector 46 disposed on the end portion thereof so that both units can be intercommunicated. The controller 40 receives its drive power from a low-voltage power supply 43, which converts 100-V commercial power supply to voltage levels required by the various units.
In the conventional laser beam printer 1, a unit for writing an image onto the photoreceptor drum using the laser scanner is constructed as shown in FIG. 6. In the example shown in FIG. 6, a laser beam Rb outputted from a laser oscillator 51 is modulated into a dot pattern to be recorded at a modulator 52 based on a signal applied from the recording control section 50. The modulated laser beam Rb is deflected toward the photoreceptor drum 10 by the polygon mirror 4 rotating at a high speed, and scans positions a, b and c shown in FIG. 6 in that order. Further, at the scanning start point is a beam detector 53 such as a PIN diode that outputs an electric signal upon reception of a beam so that a laser beam whose optical path has been bent by a reflecting mirror 54 can be detected.
When the beam detector 53 detects a beam, it outputs a detection signal, and modulation of the laser beam Rb is started in accordance with information to be recorded with a signal generating timing occurring every scanning operation as a reference. That is, the above detection signal is used as a horizontal synchronizing signal (main scanning direction). As shown in FIG. 7, based on the signal outputted at the scanning start point a, scanning is effected on the photoreceptor drum from the writing start point b to the writing end point c to thereby form a latent electrostatic image on the photoreceptor drum.
By the way, while the conventional laser beam printer is operated at a single scanning density, recent arts featured by combining a plurality of office machines and equipments as well as developments in information processing systems have led to come into being, e.g., a system combining an image reader for reading data at 400 dpi (dots/inch) and a text editor for handling 300 dpi font data together. Consequently, as a printer accommodating such a system, a laser beam printer having a plurality of scanning densities has been called for. In such a laser beam printer, a technique for switching the beam intensity in accordance with the scanning density may be employed to optimize each of images having different scanning densities. Further, in the laser beam printer, there is a tendency to use higher scanning densities; the density range of 240 to 300 dip in early versions of constant speed laser beam printers is increased to that of 300 to 400 dpi in current mainstay printers. A recent laser beam printer has a density of as many as 600 dpi. The laser beam printers having higher scanning densities are generally designed to reduce not only the beam diameter but also the beam intensity to obtain beams narrower than the conventional beams. Thus, as the scanning density is increased, the beam intensity is reduced.
On the other hand, there is a tendency that the sensitivity of the photoreceptor drum is improved, so that less light is required for forming an image on the photoreceptor drum. It is apparent from these considerations that the beam intensity required for scanning the photoreceptor drum surface is reduced to a considerably low degree compared with that in the conventional case. That is, irradiation of a beam whose intensity is more than required to form a latent electrostatic image on the photoreceptor drum may not only impair the printing quality but also deteriorate the performance of the photoreceptor drum itself. However, if the output of the laser beam is decreased to a level required by the photoreceptor drum, the output level of the beam detector for obtaining the horizontal synchronizing signal is reduced or a like problem is additionally addressed.
To overcome the above problems, a means shown, e.g., in Japanese Patent Unexamined Publication No. Sho. 59-146017 has been proposed. In the scanner of the above system, an ND filter is arranged at a position toward the photoreceptor drum so that the beam irradiated to the beam detector becomes more intense than that irradiated to the photoreceptor drum. That is, the beam intensity to be irradiated onto the photoreceptor drum can be lessened. However, in the case where the filter shown in the conventional system is provided, the scanner can meet the need for increasing the scanning density, but cannot be applied to an apparatus having a plurality of scanning densities. Further, as disclosed in the above-mentioned publication, to obtain an adequate response with a small quantity of light when a horizontal synchronizing signal is generated, the sensitivity of the beam detector must be improved to a significant degree. However, when the sensitivity of the beam detector is improved, the beam detector is subjected to noise such as electrostatic noise and power supply noise, and any disturbance of the image signal may often result in abnormal output images and defective operations of the units. To overcome the above problems, it is necessary to provide a proper quantity of light to both the beam detector from which a timing reference signal is obtained and a light-receiving means of the photoreceptor drum.